PROJECT SUMMARY AND ABSTRACT A comprehensive understanding neuronal cell type diversity is an essential guide to selective manipulation and illuminating cell type specific functional contributions toward health and disease. Accordingly, the Brain Initiative Cell Census Network (BICCN) is unifying the efforts of laboratories with unique expertise in anatomy, genetics, electrophysiology, and function to classify neurons and create a common 3D atlas with integrated cell type data. To this end, our proposed collaboratory aims to anatomically characterize neuronal cell types of the mouse limbic system. Using mesoscale quadruple retrograde tracing, we will initially characterize cell types based on the anatomical location of their connectional start and end points [e.g., ACB(contralateral)?BLAa?ACB(ipsilateral)]. A two-step cre-dependent AAV tracing strategy using advanced viral tools will subsequently validate and refine specific axonal projections, collaterals, and projection fields [e.g., ACB/X/Y?BLAa?ACB/X/Y]. Injections of G-deleted rabies in CLARITY-processed tissue will label morphological features of cell types. Cre-dependent TRIO viral tracing will determine discrete inputs to each cell type, providing deeper characterization of connectivity. Novel TRIO using flp recombinase in cre- dependent mice will define projection patterns of genetically-defined cell types. Newly constructed AAV and rabies viruses tagged to spaghetti monster fluorescent proteins, applied in combination with Expansion Microscopy and multiphoton imaging, will determine the spatial organization of different synaptic inputs to the cell types. Collectively, experiments will reveal cell type anatomic location, morphology, and comprehensive connectivity. Initial efforts will focus on the limbic system, with the design extensible to neuronal characterization of the entire brain. A web-based visualization platform will be developed to enable viewing and analysis of cell type anatomy data in 2D and 3D. An online visualization tool similar in function to our iConnectome viewer will present quadruple retrograde and TRIO tracing images. Digitized, reconstructed quadruple retrograde, cre-AAV, and TRIO labeling will be placed atop the Allen Reference Atlas (ARA) to create an online 2D connectivity map, allowing easy comparison of cell type specific inputs and outputs. Common Coordinate Framework (CCF) registration and reconstruction of cre-AAV labeling experiments will provide the cell type specific 3D context of projections, with input and morphological information integrated into the viewer. An interactive, weighted and directed matrix will present an intuitive visualization of all connectivity data. 3D reconstructed neurons will also be hosted on Neuromorpho.org for interspecies comparison. Our current informatics pipelines will be extended and optimized to support the proposed viewer features. We expect our technologies to elucidate diverse cell type specific networks and provide foundations for the overarching goal of the BICCN of creating a comprehensive 3D cell type atlas.